Pearlescent pigments comprise a platelet-shaped substrate of low refractive index, such as mica, and one or more highly refractive oxide layers. The highly refractive oxide layers used most frequently are titanium dioxide and iron oxides. The optical effect of pearlescent pigments is based on interference effects and their platelet-shaped structure. As a result of their structure, the pearlescent pigments have an orientation in the application medium that is parallel to the substrate. The lamellar structure of the pearlescent pigments oriented in the application medium in turn causes the effect-imparting properties thereof, such as the perception of interference colors and a color flop. The specific optical effect is substantially determined by the type and thickness of the oxide layer and the particle size and particle size distribution of the pearlescent pigments. But due to their platelet-shaped structure, pearlescent pigments are particularly sensitive to the influence of mechanical forces and shearing forces. Extremely vigorous shear stresses can damage the highly refractive oxide coatings on their substrate by causing fracture thereof and thereby adversely affecting the effect-imparting properties. This is manifested, for example, in their susceptibility to damage when pearlescent pigments are incorporated into plastics materials or the general necessity to work pearlescent pigments into wet paint in a gentle manner, in contrast to standard colored pigments. When incorporated into plastics, pearlescent pigments are extruded into a plastic melt. Flaking of the oxide coatings leads to a reduced transparency of the thus pigmented plastics material and to reduced luster thereof.
However, unlike colored pigments, pearlescent pigments cannot be incorporated into the basecoat of powder-based coating materials by extrusion with subsequent grinding in a pinned-disk mill. This would usually comminute them in such a way that the characteristic optical effects would be virtually completely lost.
Another critical aspect relates to the weather stability of pearlescent pigments. Due to the photocatalytic activity of titanium dioxide, the binder of the topcoat undergoes degradation, for example, in automobile lacquers. Pearlescent pigments coated with titanium dioxide must therefore be provided with suitable protective layers. However, protective layers that also improve the mechanical properties of the pigment have hitherto been difficult to find.
For a relatively long time, pearlescent pigments have therefore been provided with purely inorganic or purely organic three-dimensionally crosslinked coatings. These coatings usually serve as protective layers, but they also have a mechanically stabilizing effect. Furthermore, pearlescent pigments used in powder coating materials can be made suitably electrostatically chargeable by using dielectric coatings of this type.
The advantageous properties of the purely inorganic or purely organic three-dimensionally crosslinked coatings of pearlescent pigments must be distinguished from various other surface treatments. Surface treatments of this type always aim at improving application properties as influenced by the surface chemistry of the pearlescent pigments.
Thus, an organic surface modification consisting of, for example, inorganic coatings on pearlescent pigments has been performed thus far only in the form of a surface modification. DE 198 20 112 A1 describes reactive organic orientation aids, one functional group of which can chemically bind to the surface of a pearlescent pigment and the other functional group of which can bind to the coating material. The organic orientation aids are applied to pearlescent pigments, which have been coated with inorganic oxide layers or organic polymer layers. According to the teaching of DE 198 20 112 A1, however, no mixed layer of an inorganic oxide/hydroxide and an organic oligomer and/or polymer is formed. The organic orientation aids used merely change the surface properties of the pearlescent pigments. This involves achieving the best possible attachment of the pigment to the binder of the coating material. This favorably influences the orientation of the platelet-shaped pearlescent pigment in the coating material which is very essential to the optical effect and other technological application properties such as condensation water climate stability and dispersability in the coating system.
DE 196 39 783 A1 describes modified pearlescent pigments based on a platelet-shaped substrate which is coated with metal oxides and which contains on the topmost metal oxide layer a covering layer composed of at least two oxides, oxide mixtures or mixed oxides of silica, alumina, cerium oxide, titanium oxide, or zirconium oxide and a water-based oligomeric silane system.
“An oligomeric silane system” in this case means a system of different organofunctional silanes, which are linked to one another via inorganic —Si—O—Si-units. But the aforementioned document does not describe oligomers in which the organic functions of the silanes are covalently bonded to one another. Silane systems of this kind therefore cannot be used to form an organic oligomer and/or polymer network.
EP 632 109 B1 describes a pearlescent pigment coating comprising at least three layers. The first layer consists Of SiO2. The second layer consists of at least one hydroxide or hydrated oxide of the elements cerium, aluminum, or zirconium. The third layer consists of one or more oxides/hydroxides of the elements Ce, Al, or Zr and an organic coupling agent. Organically modified silanes are mentioned inter alia as coupling agents. Coatings of such kind give rise to weather-resistant pearlescent pigments having improved application properties. It is clear from EP-A-0 268 918 and EP-A-0 342 533 that the weather resistance, that is to say, the suppression of the photoactivity of the titanium dioxide layer, is substantially the result of deposition of oxides/hydroxides of the elements Ce, Al, or Zr.
The coatings further described in DE 196 39 783 A1 and EP 632 109 B1 act as a surface modification. This surface modification aims to retain, on the one hand, advantages in terms of technological application properties, such as improved pourability and, on the other hand, very good dispensability, color properties, low formation of micro-bubbles, luster, and stability in water-thinnable coating systems. But the document does not describe mixed layer precipitation, in which the organic components are oligomerized/polymerized with each other and thus result in the formation of an organic network within the inorganic network.
Pearlescent pigments can be improved in very many ways with regard to their application properties with the aid of a coating comprising SiO2 or other inorganic materials and are thus adapted for a wide variety of applications. The disadvantage of these purely inorganic coatings, however, is their intrinsic brittleness. It has been found that high mechanical stresses can damage these layers, which results in a loss of the desired properties.
Mechanical damage of the protective layer can likewise occur when pearlescent pigments are dispersed in a coating system too vigorously or when pearlescent pigments are dispersed in extruders for applications in plastics materials.
Frequently, optical losses caused by mechanical damage to the pearlescent pigments can be found in particular when pearlescent pigments are used in rigid plastics such as polycarbonate.
EP 515 602 (cf. DE 4 039 593) describes surface-modified platelet-shaped substrates in which organic aluminum and/or silicon compounds are anchored to the particles with the controlled application of moisture or heat. Crosslinking of the organic components after deposition onto the pigment surface is not described in the aforementioned document. Iron oxide pearlescent pigments treated according to this method are reported to have greater protection from abrasion. In contrast, EP 1 203 794 mentions that despite the stabilizing aftercoating, which is composed of 3 layers and is described in EP 0 632 109 in detail, the pearlescent pigments, especially those based on mica and coated with iron(III) oxide, are unsuitable or only limitedly suitable for use in all fields involving extreme stresses. According to the teaching of EP 1 203 794, mechanical stability can be improved only in a very complex method by the deposition of an additional, adhesion-promoting fourth layer.